The present invention relates generally to improvements in the control of the operation of an electrostatic copying machine and it relates particularly to an improved method and apparatus for fixing toner images in accordance with the thickness of the copy paper which is fed to a fuser at constant paper feeding speed.
Sheets of copy paper employed in conventional electrostatic copying machines are generally limited to a narrow range of thicknesses (weights), for example, to the range of about 50 to 85 g/m.sup.2. The conventional office paper is included in this range. In recent years, however, there has been a demand for electrostatic copying machines which can be used with thicker papers weighting 100 to 160 g/m.sup.2, such as postal cards, labels, offset master sheets, etc., and such machines have become commercially available. The use of thick paper poses problems in the fixing of the copy as well as in the path of transport of the copy paper and in the image transfer station. In thermally fixing toner to a copy sheet for the development of a latent image, it is necessary to fuse the toner and heat the copy sheet to some extent at the same time. Accordingly, when the toner is to be fixed to the same degree on a copy sheet of conventional thickness and on a thicker copy sheet of different heat capacity, the latter requires a larger amount of heat in accordance with the difference in thickness. To meet this requirement, it has heretofore been a practice to set the fixing unit at a higher temperature only when using copy sheets of greater thickness, or to set the unit to a constant temperature at which the toner images can be fixed to thick sheets. With such a method, however, the fixing unit is not satisfactorily operable for thick copy sheets unless the unit has a heater of increased power capacity to compensate for a reduction in the temperature of the unit when continuously producing a plurality of copies. Especially in the case of copying machines adapted for a relatively short copy processing time wherein the copy processing time means the period of time consumed in processing each copy during a copying operation, for example, of about 20 to 30 copies per minute, it is almost practically impossible to restrict the overall power consumption to the limit of the conventional household power supply (100 V, 15A in Japan; 115 V, 15A in U.S.). Additionally, the latter of the two methods mentioned above is uneconomical since the fixing unit, when operating for copy sheets of conventional thickness, consumes a larger quantity of heat than is needed. On the other hand, a copying machine has been proposed which is so controlled that the copy processing time is reduced in accordance with a reduction in the temperature of the fixing unit to maintain the unit at least at a temperature below which improper fixing will result (Japanese Laid Open Patent Application Tokkaisho No. 54-80135). Another copying machine has also been proposed in which a copying start instruction is given only when the detected temperature of the fixing unit is no lower than a reference temperature to permit no copying operation at temperatures that would cause improper fixing (Japanese Laid Open Patent Application Tokkaisho No. 54-109446). With these machines, however, the variations in the temperature of the fixing unit, namely, the drop and recovery of the temperature are allowed to spontaneously occur and influence the copying processing time interval, producing varying copy processing times. When such machine is featured by a short copying processing time, the performance of the machine is still unsatisfactory in that the copying operation requires indefinite varying times.